Definition 3.1 
[1]} Let \(U\)  denote an open subset of \(\mathbb {R}^n\)  and \(k\ge 1\)  an integer. An expression of the form:
\(\mathbf {\mathcal {L}}(D^k \mathbf {u}(x), D^{k-1} \mathbf {u}(x), \dots , \mathbf {u}(x), x) = \mathbf {0}\quad \forall x\in U\) 
